This invention generally is directed to synthetic rubber elastomers, methods for their manufacture and use, and to physically and mechanically improved articles manufactured therefrom. More specifically the present invention provides new synthetic rubber elastomer compositions, articles and methods of making same which embody the desirable physical qualities normally associated with natural rubber latex in addition to markedly reduced allergenicity and significantly improved chemical and physical properties including solvent resistance, elasticity and resilience.
Broadly stated, the present invention provides new enhanced synthetic rubber elastomers having unique combinations of physical, chemical, and mechanical properties which make them particularly well suited for replacing natural rubber latex in articles traditionally made from natural rubber latex. More specifically, the present invention is directed to functionally enhanced synthetic rubber elastomers incorporating unique cross-linkages such that the compositions function as elastomeric materials having aspects of thermoset material stability and function. These unique materials have been optimized to function as improved replacements for cured natural rubber latex while maintaining, and even surpassing the beneficial physical, chemical, and mechanical properties of natural rubber latex. Moreover, in addition to being readily applicable to the majority of existing latex article manufacturing processes, the synthetic rubber elastomer of the present invention possess the added benefit of being compatible with continuous manufacturing techniques including extrusion, blow forming, injection molding, rolling and sheet formation.
Articles manufactured from natural rubber latex exhibit a variety of desirable properties including resistance to creep (resisting the undesirable elongation of a material under constant stress), compression resistance (the ability of an article to return to its original size and volume after squeezing), elasticity, solvent and plasticizer resistance, and overall biocompatibility. Unfortunately, a primary drawback associated with natural rubber latex articles is the growing number of people that are allergic to them.
Early attempts at producing articles made from alternative non-natural rubber latex materials have been generally successful though not without their associated problems. Substitute artificial or synthetic latex materials are relatively expensive when compared to natural rubber latex and in some cases are considerably more expensive. More importantly, in most applications their physical, mechanical, and chemical properties are markedly inferior to those of natural rubber latex. For example, some artificial rubbers lack sufficient elasticity or strength to function effectively as gloves. Some are affected by solvents making them difficult to use around alcohols or naturally occurring oils which can soften and degrade the material. Others exhibit deforming material creep when subjected to constant stress. This can result in sagging and bagginess that make such materials inappropriate for inflation balloons or similar structures. Other latex replacements possess poor compression resistance and permanently deform when subjected to compressive stress. These inferior properties can make it difficult to package, store, or use articles manufactured from these alternative materials without damage.
One method for producing natural rubber latex substitutes is described in U.S. Pat. Nos. 5,112,900 and 5,407,715 both issued to Buddenhagen et al. (collectively referred to hereinafter as xe2x80x9cthe Buddenhagen patentsxe2x80x9d). The Buddenhagen patents describe a method for making elastomeric compounds composed of at least two styrene-ethylene/butylene-styrene (S-EB-S) tri-block co-polymers that are dissolved in a suitable solvent and mixed with a plasticizer such as mineral oil. The resulting emulsion is then used to form medical devices using a xe2x80x9cdippingxe2x80x9d technique followed by air-drying. The principal drawback, other than manufacturing costs, to Buddenhagen materials is that post manufacturing crosslinking is not possible. As a result, articles formed using the Buddenhagen technique are subject to creep and deformation.
Another method for producing natural rubber latex substitutes using hydrogenated tri-block thermoplastic elastomers (HTPEs) is discloses in U.S. Pat. No. 5,264,488 issued to Takeuchi et al. Nov. 23, 1993 (the xe2x80x9cTakeuchi patentxe2x80x9d). However, as in the cases with Buddenhagen, the synthetic rubber elastomers disclosed in the Takeuchi patent were not crosslinked and therefore remain subject to creep and general physical instability.
Accordingly, it is a primary objective of the present invention to provide an effective material suitable for use as a replacement for natural rubber latex in the production of commercial and medical products. Concomitant with this objective is providing elastomeric replacement materials for natural rubber latex that can be incorporated into most existing continuous manufacturing processes without significant modification or expense.
It is an additional objective of the present invention to provide alternative materials, especially for medical devices and products, enhancing their physical, chemical, and mechanical properties and thus the functionality of these devices and products.
These and other objects are achieved by the present invention which provides novel biocompatible, nonallergenic synthetic rubber elastomer, methods for producing these compounds and articles made therefrom. The novel synthetic rubber elastomers made in accordance with the teachings of the present invention provide functionally enhanced elastomers that, as compared to natural rubber latex, are less allergenic, possess increased elastomeric resilience, decreased creep, and increased resistance to oils, lipids and organic solvents while simultaneously eliminating many of the adverse properties which can significantly limit the natural product""s utility.
Articles made from the synthetic rubber elastomers of the present invention possess excellent elasticity, exceptional resilience, exhibit minimal creep and resist swelling when exposed to oils, lipids and organic solvents, yet remain non-toxic. This unique combination of physical and biological properties results in a material that is ideally suited for medical devices and other products including those which may contain natural rubber latex.
The synthetic rubber elastomers of the present invention are generally made using a first component composed of at least two hydrogenated tri-block thermoplastic elastomers (the xe2x80x9cHTPE componentxe2x80x9d) and a second component containing a cyclic unsaturated polyolefin crystalline polymer (the xe2x80x9cCPP componentxe2x80x9d), such as, but not limited to, cyclic polybutadiene, cyclic polyisoprene and cyclic polymers of 1,2 dimethylene cyclohexane. The two components are mixed and used to form a synthetic rubber elastomeric article. In another embodiment of the present invention a third component containing anti-oxidants and anti-ozonates can be added prior to forming the synthetic rubber elastomeric article. In yet another embodiment of the present invention a fourth component containing reinforcing materials can be added. Manufacturing procedures include, but are not limited to extrusion, injection molding, compression molding, calandering, and blow molding. The articles thus formed are cross-linked during a post-extrusion process in order to modify the synthetic rubber elastomer""s properties.
In one embodiment of the present invention, the HTPE component is composed of at least two different HTPE polymers of the styrene-ethylene/propylene-styrene (S-EP-S) or styrene-ethylene/butylene-styrene (S-EB-S) type. Generally, HTPE polymers are selected to provide the HTPE component of the present invention with at least one low molecular weight HTPE and at least one high molecular weight HTPE.
The synthetic rubber elastomers produced in accordance with the teachings of the present invention combined the HTPE with the CPP component. The CPP component provides crosslinkable unsaturated groups to increase HTPE entanglement. The extent of entanglement present in the HTPE component is a function of the number, type and location of the double bonds within the CPP component and contributes to the synthetic rubber elastomer""s character and functionality. This, in turn, is controlled by the raw materials used and reaction conditions selected utilizing the teachings of the present invention. Thus, utilizing the teachings of the present invention, the HTPE component of the present invention can be modified or designed to exhibit a wide range of physical properties.
Modification of the HTPE component is further accomplished through the addition of additional intermediate weight products or non-hydrogenated tri-block analogues. Moreover, xe2x80x9cstarxe2x80x9d or branched analogues of the non-hydrogenated tri-block thermoplastic elastomeric polymers can be incorporated to modify chain entanglement and improve resilience. The extent of cross-linking present in the synthetic rubber elastomer of the present invention determines the final physical properties exhibited ranging from thermoplastics to thermoset resins. Consequently, precise control of the properties and functions of the materials and articles of the present invention can be achieved. This provides yet another advantageous aspect of the present invention and significantly increases the options available to materials engineers, product designers, and end users.
The synthetic rubber elastomers of the present invention so produced can then be used to manufacture articles such as balloon catheters or surgical gloves which can then be cured and cross-linked using methods such as, but not limited to, electron beam radiation, gamma radiation, or chemical vulcanization.
Further objects and advantages of the methods, compositions and articles of manufacture produced in accordance with the teachings of the present invention, as well as a better understanding thereof, will be afforded to those skilled in the art from a consideration of the following detailed explanation of exemplary embodiments thereof.